1. Field of the Invention
The present invention relates to a carrier providing a charge to a toner by frictionizing the toner, a two-component developer including a toner and the carrier, and an image forming apparatus such as copiers and laser printers and a process cartridge using the developer.
2. Discussion of the Background
An electrophotographic image forming method typically forms an electrostatic latent image on a photoconductive image bearer, provides a charged toner to the electrostatic latent image to form a visual image, transfers the visual toner image onto a transfer medium such as papers, and fixes the visual toner image on the transfer medium with a heat, a pressure or a solvent vapor, etc.
The electrophotographic image forming method is broadly classified as a two-component developing method wherein a toner is charged by mixing the toner with a carrier and a one-component developing method wherein a toner is charged without using a carrier.
One-component developing methods are broadly classified into magnetic developing methods and non-magnetic developing methods, depending on whether a toner is magnetically borne by a developing sleeve.
Conventionally, two-component developing methods have good toner charge stability and buildability and stably produce quality images for long periods. These devices are mostly used for printers, copiers, and complex machines which are required to have high-speed printability and quality image reproducibility. One-component developing methods are mostly used for small printers and facsimiles which are required to be space-saving and low-cost.
Recently, the demand for color images has increased significantly, therefore, the demand for stable, high-quality images has also increased significantly.
Japanese Laid-Open Patent Publication No. 58-184157 and Japanese Patent Publication No. 5-8424 disclose two-component developing methods using a magnetic carrier, wherein the carrier has a small particle diameter and a thin magnetic brush formed on the developer such that a latent image is more finely developed to produce high-quality images.
However, the magnetic carrier having a small particle diameter has a low magnetization per particle, and, therefore, the magnetic binding force attracting the particle to a magnetic sleeve is reduced, resulting in carrier transfer, or adhesion, of particles onto the image bearer.
To prevent carrier adhesion of small particles of the magnetic carrier, in developing methods of feeding a developer by rotating a magnet included in a developing sleeve, Japanese Laid-Open Patent Publication No. 2000-137352 discloses a method of setting a lower limit of the carrier saturation magnetization and Japanese Laid-Open Patent Publication No. 2000-338708 discloses a method of setting a lower limit of a product between a particle diameter and a residual magnetization of the magnetic carrier.
In other words, these methods prevent feeding the carrier having a small magnetic binding force. However, as an electrostatic element is added to the carrier in the image developer, a desorption force is occasionally higher than the binding force and carrier adhesion cannot sufficiently be prevented.
In Japanese Laid-Open Patent Publication No. 2000-137352, a saturation magnetization in an electric field of 10,000 Oe is used, such a high electric field is not used in conventional electrophotographic image developers and the carrier adhesion cannot always be sufficiently prevented even when such a method is used.
Japanese Laid-Open Patent Publication No. 4-145451 discloses a method of removing carrier particles having a specific low saturation magnetization, a small particle diameter, and a small specific gravity regardless of their particle diameters to prevent carrier adhesion. However, in Japanese Laid-Open Patent Publication No. 4-145451, although the final properties of the carrier are not disclosed, sufficient prevention of carrier adhesion is not expected at present since further uniformity of the carrier particles is required.
Japanese Laid-Open Patent Publication No. 2002-296846 discloses a method of specifying a volume-average particle diameter, a particle diameter distribution, a magnetization in a magnetic filed of 1,000 Oe of a core material of a carrier, and a magnetization difference between the carrier and scattered materials to prevent carrier adhesion.
It can be supposed that the method of Japanese Laid-Open Patent Publication No. 2002-296846 has a specific prevention effect for carrier adhesion because of the absence of particles having a small magnetic binding force.
Carrier adhesion is thought to occur due to differences on how individual carrier particles react to external forces, and, particularly in developing methods using a magnetic brush, to differences on magnetic binding forces of individual carrier particles.
However, although Japanese Laid-Open Patent Publication No. 2002-296846 specifies a magnetization ratio between carrier and scattered materials, it is silent on how individual carrier particles are directly involved in carrier adhesion. Thus, such a method is still insufficient to produce high-quality images while preventing carrier adhesion.
Further, in Japanese Laid-Open Patent Publication No. 2002-296846, properties of the carrier core material are controlled to prevent carrier adhesion and other effects. However, as the carrier properties largely depend on mechanical, chemical, electrical, physical and thermal properties of a coat layer of the carrier besides the properties of the core material, control only of the core material properties does not always result in sufficient control of carrier properties.
Particularly, as image quality and stability largely depend on surface properties of the carrier used in the image forming apparatus, carrier particles having a coat layer are needed for better image quality.
Recently, in consideration of environmental protection, units using one-component developing method are mostly recycled and reused, while, at the same time, two-component developers are preferred because of their extended useful life.
On the other hand, in order to decrease energy consumption, toner image fixing temperatures continues to decrease, resulting in toners being easily deformed and firmly fixed at a lower temperature.
The two-component developers are deteriorated because of (1) carrier surface abrasion; (2) separation of a coat layer on the carrier surface; (3) carrier crushing; and (4) deterioration of chargeability, transfer from a desired resistivity of the carrier, and generation of foreign particles such as broken pieces and abrasion powders accompanied by fixation (spent) of a toner on the carrier. These cause image quality deteriorations such as deterioration of image density, foggy background, and deterioration of image resolution as well as physical and electrical damages to image carriers.
Many suggestions having some benefit have been made to solve the above-mentioned problems and improve durability of the carrier. As to suggestions focusing on a coat layer of a coated carrier, i.e., a carrier having a coat layer on a surface of its core material, several techniques have been disclosed. For example, Japanese Laid-Open Patent Publication No. 8-6308 discloses a carrier having a coat layer, which is made of a hardened polyimide substance, including specifically bimaleimide, to improve environmental stability, and prevent foggy background and separation of the coat layer. Japanese Patent No. 2998633 discloses a carrier having a resin coat layer wherein a matrix resin that includes dispersed resin particles and electroconductive fine particles is used to minimize toner consumption. Japanese Laid-Open Patent Publication No. 9-311504 discloses a carrier having a coat layer formed of a phenol resin including a hardened amino group on a surface of a spheric complex core particulate material formed of an iron oxide powder and a phenol resin. In this carrier, contents of the iron oxide powder and the amino group are specified in order to obtain a stable frictional charge and durability. Japanese Laid-Open Patent Publication No. 10-198078 discloses a carrier having a coat layer formed of a matrix resin including dispersed resin fine particles and electroconductive fine particles, wherein the matrix resin includes not less than 10% of components of a binder resin of the toner to decrease the influence of toner spent on chargeability. Finally, Japanese Laid-Open Patent Publication No. 10-239913 discloses a carrier having a coat layer formed of a polyimide resin having a repetition group including a diorganosiloxy group and a compound including two or more epoxy groups in a molecule, resulting on a stable charged amount.
However, the level of performance of these conventional techniques is not acceptable at present, particularly considering the fact that further decreases in fixing temperature and higher carrier longevity are expected. In Japanese Laid-Open Patent Publication No. 8-6308, Japanese Patent No. 2998633, and Japanese Laid-Open Patent Publication Nos. 9-311504 and 10-239913, the matrix resin occupies most of the carrier surface alone and the toner fixation mostly depends on the surf ace status of the matrix resin. Therefore, a sufficient solution to the problem at hand is not always obtained.
In Japanese Laid-Open Patent Publication No. 10-198078, which uses a toner having a low temperature fixability, the same components on the surface of the carrier as those of the toner binder resin tend to lead to toner fixation and the toner is not occasionally stably charged from the beginning of the toner agitation process.
Many suggestions of forming a coat layer with a silicone resin having a comparatively low surface energy have also been made. However, silicone resins do not adhere to the core carrier material due to their low surface energy.
Japanese Laid-Open Patent Publication No. 58-108548 discloses a carrier coated with a specific resin; Japanese Laid-Open Patent Publications Nos. 57-40267, 58-108549, 59-166968 and 6-202381 and Japanese Patent Publication No. 1-19584 disclose carriers coated with specific resins including various additives; and Japanese Patent No. 3120460 discloses a carrier coated with a specific resin having an additive adhered on its surface. Japanese Laid-Open Patent Publication No. 8-6307 discloses a carrier mainly coated with a benzoguanamine-n-butylalcohol-formaldehyde copolymer. Japanese Patent No. 2683624 discloses a carrier coated with a cross-linked resin between a melamine resin and a acrylic resin. However, these carriers do not have sufficient durability yet.
To improve charged amount instability of the carrier accompanied by the spent toner on the surface thereof and resistance variation due to an abrasion of the coated resin, Japanese Laid-Open Patent Publications Nos. 2001-117287, 2001-117288 and 2001-188388 disclose a carrier coated with a thermoplastic resin and a carrier coated with the thermoplastic resin having a larger particle diameter than that of the binder resin.
Japanese Laid-Open Patent Publication No. 9-319161 discloses a method of dispersing fine particles of a specific thermoplastic resin in the matrix resin of the coat layer as another method of maintaining the coat layer properties of the carrier, particularly the chargeability thereof. By this method, even an abraded coat layer have equivalent properties to those of the initial coat layer. However, the method does not sufficiently decrease the abrasion.
Even the method in Japanese Patent No. 2998933, wherein an electroconductive fine powder is dispersed at the same time in addition to the specific thermoplastic resin, does not sufficiently decrease the abrasion problem either.
As mentioned above, trials to fundamentally improve carrier adhesion in a two-component developer capable of stably producing high-quality images have not yet been made having the various binding forces and desorption forces applied to the carrier particles in image developers within a desired range. This still remains a difficult challenge.
Further, preventing carrier adhesion and abundantly and softly forming or properly renewing a developer brush on a developing sleeve to properly feed toner onto an electrostatic latent image bearer and produce high-quality images with high image density and without background fouling still remain a difficult problem.
Because of these reasons, a need exists for a carrier producing high-quality images without carrier adhesion.